In the copending patent application entitled Propulsive Thrust Ring System filed in the United States Patent and Trademark Office on Jun. 5, 1991 bearing Ser. No. 711,622, now U.S. Pat. No. 5,292,088, a propulsive thrust ring system is described with which vortices from a propeller are converted to useful mass flow. This application and any patent as may issue therefrom is incorporated herein in its entirety by reference thereto.
U.S. Pat. No. 4,506,849 of which I am the inventor is entitled Helicopter Rotor Thrust Ring describe use of a single thrust ring around a tail rotor. Both crossflows from the main rotor and from translational flight are used to enhance thrust from the thrust ring. The area of the single thrust ring required to adopt it for sufficient axial propulsion augmentation would impose too much drag to operate successfully in an axial, high-speed mode.
Townend rings (an early method of streamlining radial aircraft engines) have been used to enclose aircraft propellers to augment thrust. This is an aircraft variant of the shrouds of U.S. Pat. Nos. 3,722,454 and 3,969,944. The same objections which apply to aircraft also apply to water applications. While these Townend ring boost static ground thrust at moderate aircraft translational speeds, drag soon equals the increased thrust effect to reduce the net effectiveness to zero.
Transport aircraft use propulsive fans which are included within a duct/cowling system for noise control. Thrust enhancement by the distribution of negative air pressures on the cowl lip as well as the elimination of the blade tip vortices when the fan blades are located in the duct are claimed. Variations of duct lip pressures with translational and cross flow cause changes in internal duct air velocities, both in direction and magnitude to effect internal fan face velocity distributions. This upsets the match of the required fan blade twist to the duct internal radial air flow distribution and reduces efficiency. To eliminate the tip vortices, the fan blade tips need to be very close to the duct wall (less than 0.5% of the radius) with subsequent clearance problems. The small clearance creates a major disadvantage because of the additional weight required to stiffen the duct structure to prevent contact by the fast-moving blades.